Chaotic dynamics in a nonautonomous Dicke model without the rotating-wave approximation

The properties of a system consisting of two-level atoms interacting with a mode of the electromagnetic field in a cavity are considered for the case when the cavity detuning or the coefficient of the atom-field interaction is modulated. The consideration is performed with account taken of the Hamiltonian terms that are neglected in the rotating-wave approximation. It is shown that in the semiclassical equations for such a model, the effect of extension (compared to the autonomous system) of the range of variation of the quantity characterizing the number of photons can manifest itself; in this case, the energy oscillations have a chaotic character. The dependence of this phenomenon on parameters characterizing the model is studied. It is numerically demonstrated that with account taken for the relaxation, the system studied can have attractors different from the equilibrium positions, i.e., the number of photons in the mode does not tend to a constant value. The limits of validity of the rotating-wave approximation in the parametrically perturbed Dicke model are discussed.     

Impulsive generation of coherent magnons by linearly polarized light in the easy-plane antiferromagnet FeBO3

Polarization-dependent excitation of coherent spin precession by 150 fs linearly polarized laser pulses is observed in the easy-plane antiferromagnet FeBO3. We show that the mechanism of excitation is impulsive stimulated Raman scattering. This process is shown to be determined not only by the magneto-optical constants of the material, but also by the properties of the spin precession itself. Though carrying no angular momentum, the linearly polarized laser pulses act on the spins as effective fields that can be considered as an ultrafast inverse Cotton-Mouton effect.     

The role of magnetization compensation point for efficient ultrafast control of magnetization in Gd<Subscript>24</Subscript>Fe<Subscript>66.5</Subscript>Co<Subscript>9.5</Subscript> alloy

The ability of a femtosecond laser pulse to manipulate and reverse the magnetization in a ferrimagnetic Gd₂₄Fe_66.5Co_9.5 thin film was studied experimentally as a function of temperature. For a fixed energy of the laser pulse, the dynamics of magnetization showed different behavior depending on whether the sample temperature was below or above the magnetization compensation point (T _M ). The conditions for full ultrafast demagnetization and magnetization reversal were easily achieved below T _M , while the same laser excitation caused just 50% demagnetization above T _M . This interesting change in magnetization dynamics is qualitatively explained in terms of effective changes in the magnitudes of magnetizations of atomic sublattices.     

Role of magnetic circular dichroism in all-optical magnetic recording

Using magneto-optical microscopy in combination with ellipsometry measurements, we show that all-optical switching with polarized femtosecond laser pulses in ferrimagnetic GdFeCo is subjected to a threshold fluence absorbed in the magnetic layer, independent of either the excitation wavelength or the polarization of the laser pulse. Furthermore, we present a quantitative explanation of the intensity window in which all-optical helicity-dependent switching (AO-HDS) occurs, based on magnetic circular dichroism. This explanation is consistent with all the experimental findings on AO-HDS so far, varying from single- to multiple-shot experiments. The presented results give a solid understanding of the origin of AO-HDS, and give novel insights into the physics of ultrafast, laser controlled magnetism.     

Stress state of a piezoelectric elastic medium with an arbitrarily oriented triaxial ellipsoidal inclusion

We determine the electrostressed state of a piezoceramic medium with an arbitrarily oriented triaxial ellipsoidal inclusion under homogeneous mechanical and electric loads. Use is made of Eshelby’s equivalent inclusion method generalized to the case of a piezoelectric medium. Solving the problem for a spheroidal cavity with the axis of revolution aligned with the polarization axis demonstrates the high efficiency of the approach. A numerical analysis is carried out. The stress distribution along the surface of the arbitrarily oriented triaxial ellipsoidal inclusion is studied     

2,5‐Dihydro‐1H‐imidazole‐Based Nitroxides as Prospective Mediators in Living Radical Polymerization

A number of spectroscopic methods were applied to obtain kinetic parameters of reactions modelling the 2,5‐dihydro‐1H‐imidazole 1‐oxide mediated living polymerization of acrylates. The homolysis rate constants of alkoxyamines based on five nitroxides were measured by EPR spectroscopy at different temperatures. The recombination rate constants k_c between the corresponding alkyl radicals and the nitroxides were measured by means of laser flash photolysis. The time‐resolved chemically induced dynamic nuclear polarization (TR‐CIDNP) experiments revealed the negligible contribution of disproportionation in the recombination reaction. In addition, the thermodecomposition of alkoxyamines in the NMR probe showed the absence of intramolecular elimination of hydroxylamines from the corresponding alkoxyamines. Analysis of the kinetic parameters showed that the 2,5‐dihydro‐1H‐imidazole 1‐oxide type radicals are promising mediators for the living polymerization of acrylates and methacrylates.     

Viral capsids as templates for the production of monodisperse Prussian blue nanoparticles

The use of a viral template has allowed the synthesis of monodisperse Prussian blue nanoparticles with a diameter of 18 +/- 1.7 nm and their organization into hexagonal patterns on mica and hydrophilic carbon surfaces.     

Medium acidity and catalytic properties of composite materials based on silica and titania and powder cellulose in the presence of Cu<Superscript>2+</Superscript> ions

Composite materials (CMs) were synthesized from silica and titania xerogels. The influence of the charged state of the CM surface on complexation with Cu²⁺ ions and the catalytic activity of the obtained samples in model oxidative dehydrogenation of trimethylhydroquinone with air oxygen was analyzed. Using powder cellulose in the synthesis of CMs increased the specific surface of inorganic dioxides and lowered the negative electric potential of the CM surface. This caused a redistribution of the ratio between different types of copper structures formed in the samples during the copper sorption and led to the catalytic activity of the CMs.